Why did the guitar go to the doctor?

This joke strums up its humor through a clever bit of wordplay, relying on the double meaning of the word "fret." On one hand, to "fret" means to worry or be anxious, which is something a person might experience and thus prompt a visit to the doctor. On the other, a "fret" is a very specific, crucial part of a guitar.

On a guitar, frets are the thin strips of metal embedded into the neck that divide the fingerboard. When you press a string down behind a fret, it shortens the vibrating length of the string, producing a specific note. If one of these frets is "bad" – perhaps worn down, uneven, or loose – it can cause notes to buzz, sound dead, or even be out of tune, making the guitar quite literally "sick" and in need of a professional repair, much like a person would see a doctor for an ailment. It’s a delightful musical pun that hits all the right notes for a chuckle.